Optical devices, such as polarizers and mirrors, are useful in a variety of applications including liquid crystal displays (LCD's). Liquid crystal displays fall broadly into two categories: backlit (e.g., transmissive) displays, where light is provided from behind the display panel, and frontlit (e.g., reflective) displays, where light is provided from the front of the display (e.g., ambient light). These two display modes can be combined to form transflective displays that can be backlit, for example, under dim light conditions or read under bright ambient light.
Conventional backlit LCDs typically use absorbing polarizers and can have less than 10% light transmission. Conventional reflective LCDs are also based on absorbing polarizers and typically have less than 25% reflectivity. The low transmission or reflectance of these displays reduces display contrast and brightness and can require high power consumption.
Reflective polarizers have been developed for use in displays and other applications. Reflective polarizers preferentially transmit light of one polarization and preferentially reflect light having an orthogonal polarization. It is preferred that reflective polarizers transmit and reflect light without absorbing relatively large amounts of the light. Preferably, the reflective polarizer has no more than 10% absorption for the transmission polarization. Most LCD's operate over a broad range of wavelengths and, as a consequence, the reflective polarizer must typically operate over that broad wavelength range, as well.
In backlit displays, the reflective polarizer can be used to increase the efficiency of light utilization by reflecting the polarization of the light not transmitted by the polarizer back into the backlight. The backlight converts the polarization state of the recycled light for transmission through the reflective polarizer. This light recycling can increase total display brightness. In reflective and transflective displays, the reflective polarizer has lower absorptivity and color than most absorbing polarizers for the pass polarization of light, and can increase brightness of the display by up to 50% or more. Characteristics of reflective polarizers that are important to at least some applications include, for example, the thickness of the polarizer, the uniformity of reflection over a wavelength range, and the relative amount of light reflected over the wavelength range of interest.